1. Field of the Invention
The present invention relates to a power switching circuit for remotely activating a power supply, where the power switch is electrically isolated from the primary of the power supply.
2. Description of the Related Art
Most IBM compatible PCs have three separate physical devices, namely the keyboard, the monitor and the main computer or system unit. The system unit typically contains the processor, the data storage units, the power supply and other support circuitry. The main power switch for the computer system is typically located on the system unit for close proximity to the power supply, and to achieve as little exposure as possible of the operator to the voltages and currents associated with the power supply. This is a simple way to more easily comply with requirements promulgated by safety agencies.
The power supply includes a power transformer having a primary and a secondary side, where the primary side carries primary power comprising high voltage and current capacity that could harm the operator upon accidental contact. The secondary side carries much lower voltage and current capacity, where direct contact is relatively safe and not as harmful to the operator. The power switch must ultimately disable the circuitry on the primary side of the power supply transformer, so that placement on the system unit locates the switch near the power supply and limits the exposure of the operator to primary power only when interfacing the power switch. Otherwise, the operator interfaces mostly with the keyboard and mouse, which are directly coupled to the much safer secondary of the power supply.
Nonetheless, it is desirable to place the power switch at a more convenient location for the operator. If the switch remains on the system unit, the system unit must be conveniently located. To this end, the primary computer unit may be placed on the top of the operator's desk for easy access to the power switch, the floppy drives and other functions commonly associated with the system unit. It is common, however, to place the system unit on the floor to conserve desktop space, or when the unit is particularly designed for placement on the floor, such as tower configurations. In any event, the system unit must not be placed too far from the operator since the power switch should be readily accessible.
A better solution is to separate the main power switch from the system unit, without increasing risk of harm to the operator. Several methods have been used to solve this dilemma. Switching boxes may be purchased to handle all of the switching functions by providing a central location for all power switches, including switches for the main power supply, the monitor, and even a printer. One problem with this technique is that some switching units may still convey dangerous primary power close to the operator. Also, the switch box is often expensive, and usually consumes valuable space even if it is smaller than the primary unit.
Another solution is to electrically isolate the power switch from the power supply primary using isolation circuitry, where the isolation circuitry forms a safe boundary through magnetic, optical or even capacitive means. If so isolated, the power switch may be located at a convenient place, such as on the keyboard. Such electrical isolation is not easily achieved, however, and often adds other complications. For example, one design uses a separate power supply providing several watts of power to a separate secondary, where this secondary circuit remains powered all the time. This design requires a power transformer and optical means to cross the safety boundary. There are several problems with this technique. First, the power transformer typically operates at 60 Hertz (Hz) causing electromagnetic interference (EMI) noise problems due to the switching frequency. Thus, the separate power transformer transmits noise even when the computer system is off. Also, the power transformer consumes valuable space within the computer and is provided only at a significant cost. Furthermore, the power transformer consumes valuable energy since it is always powered on, absorbing several watts of power even while the computer is not in use and supposedly "off."
Therefore, it is desirable to provide an inexpensive way for turning on and off the power supply at a convenient location, while also providing a safety barrier between the power supply and the operator, and without consuming substantial power or generating radiated noise.